Liberal groups hoping to revive enthusiasm before November's midterm elections are encouraging their members to come to the Mall on Saturday for a rally that they expect to draw tens of thousands of people.

Their goal is to reclaim the excitement that surged among left-leaning groups after the 2008 presidential race but that more recently has belonged to tea party groups and other conservative activists. Last month, for instance, conservative commentator Glenn Beck partly filled the Mall with tens of thousands of his supporters.

The organizers of this weekend's rally, dubbed One Nation Working Together, are calling it the "most diverse march in history." The amalgam of 400 progressive groups - including environmentalists, antiwar activists, church and civil rights groups, union organizers and gay rights coalitions - is planning four hours of speeches, songs and poetry.

"We lose separately, and absent of a strategy to work together, we will continue to lose," said Phaedra Ellis-Lamkins, chief executive of Green for All, an environmental group supporting the event. "We have to be able to take critical action on all of the issues facing this country. We're at a critical moment in history, and we have the opportunity to move forward in a really significant way."

One Nation's organizers, hoping to compete with the tea party movement, will try to hold the groups together as a revived political force if their rally is successful. They are promoting the event through their network of groups, on liberal radio and on television host Ed Schultz's show.

"We aren't the alternative to the tea party; we are the antidote," said NAACP President Benjamin Jealous, who is also a lead organizer. The team that produces the NAACP's annual Image Awards show are putting together the One Nation rally.

The groups involved represent many of President Obama's core supporters, including the National Council of La Raza, the NAACP, the AFL-CIO, the Service Employees International Union and the U.S. Student Association. The rally is separate from the Democratic Party's plan to spend $50 million to try to reach those same voters, though Organizing for America, a grass-roots network that Obama's campaign created, will send out an e-mail encouraging its members to attend.

Organizers' expectations are high, though last week, leaders were still trying to raise money to bring 50,000 students to the event. A plan by Comedy Central television hosts Jon Stewart and Stephen Colbert to hold dueling rallies on Oct. 30 could appeal to some potential One Nation participants, creating competition for liberal-leaning would-be activists.

Unlike Beck's rally, in which overt politics took a back seat to religious and patriotic themes, the progressive groups will repeatedly remind attendees to vote in the midterm elections, said Arlene Holt Baker, executive vice president of the AFL-CIO.

"We have heard a lot about frustration and anger leading to apathy. What we are finding is that anger is now turning into, 'Okay, let's act,'â" she said. "We can either sit here and not move forward or we can go backward."

Holt Baker considers the rally a first step toward more closely unifying the progressive movement, which has often splintered into niche causes.

But the range of groups participating in One Nation could create some tension. Gay rights groups, such as the Human Rights Campaign, will attend alongside socially conservative faith groups, including the National Baptist Convention and the National Missionary Baptist Convention of America. Along with environmental groups, a mine workers union has also endorsed the event.

Organizers say they are focusing on three unifying ideas: jobs, justice and education. They define those ideas in a set of principles that also lays out a list of causes largely supported by liberals, such as ending discrimination in the criminal justice system, protecting Social Security, spending federal money to create jobs and improving public education.

"These are the bedrock issues that define who we are and why we march," Jealous said.